The present invention relates to television systems, and more particularly, those that transmit information that is in addition to the normal video information ("teletext").
It is known that television video signals are reproduced using a cathode ray tube (CRT) that has an intensity modulated electron beam that sweeps from left to right and top to bottom across a fluorescent phosphor coated CRT face to produce a raster which contains a picture having an intensity at each point of the raster in accordance with the intensity of the beam at each respective point. When the beam reaches the bottom of the raster, it must thereafter jump back ("retrace") to the top. During such retrace, the electron beam must be cut-off ("blanked") to avoid generating spurious picture details. The time duration of this cut-off is known as the "vertical blanking interval" (VBI) and is designated by the numerical "100" in FIG. 1. During the VBI, there occurs a vertical synchronization signal 101, that determines exactly when the beam is to start its retrace. Preceding and succeeding the vertical sync signal 101 are three horizontal lines having equalizing pulses occuring at twice the horizontal frequency in first and second equalizing pulse intervals 102 and 103 respectively.
Teletext is a system for transmitting a still picture, such as a weather map, stock market report, general news, etc. that is time-division multiplexed onto a standard television signal. Each picture is called a "page", and the totality of all pages is called a "menu". Teletext and similar data transmission systems use coded signals inserted during horizontal lines 15, 16, 17 or 18 in the VBI. These teletext signals have picture level amplitudes, i.e., amplitudes between blanking and white levels. In some receivers, vertical retrace is relatively slow and vertical blanking is insufficient so that such a teletext signal may appear on the CRT display, thereby causing interference, i.e., the sweeping CRT electron beam is still coming back to the top of the CRT when the teletext signal starts being transmitted. In general, interference can occur from the insertion of teletext signals on any line after the vertical sync pulse if blanking is very insufficient.
It is desirable to use as many lines occuring in the VBI as practical to transmit teletext since the more lines, the higher the page transmission rate. However, it has been found impractical to use more than two lines after the vertical sync pulse in each VBI due to the interference problem discussed above.
Teletext or other data signals can be transmitted during the first equalizing pulse interval 102 shown in FIG. 1 when potential interference would be out of view below the bottom of the picture. Use of three lines per field with a 100 page continuous menu would result in a maximum waiting time for a selected page of about 10 seconds as compared to 30 seconds for a one-line system. The data format can be arranged to accomodate the twice horizontal line frequency equalizing pulses.
It is therefore desirable to have a teletext system that has a high transmission rate without causing interference to the normal displayed picture.